Debris Transfer

Debris transfer occurs to some extent at most glacier snouts in the ablation area because here compressive flow causes the upward flow of debris-rich ice. Within some glaciers this process is facilitated by the development of thrusts, thrust faults or shear planes, which may transfer basal debris into an englacial, or supraglacial location. Thrusts develop in three situations: (i) in glaciers with a mixed or polythermal basal thermal regime; (ii) where glaciers flow against large subglacial bedrock obstacles or reverse bedrock slopes; and (iii) in surging glaciers.

Thrusts are most common in glaciers with a mixed thermal regime. The zone of thermal transition from warm- to cold-based ice is associated with strong compression. The warm-based ice slides over its bed, but the cold-based ice does not. This causes a down-glacier deceleration in velocity, which can generate compression beyond that which ice-creep can accommodate. As a consequence of this, a fault (thrust) may develop. These thrusts form at the base of the glacier and allow basal

Figure 7.12 Schematic summary of contemporary debris transport processes at Haut Glacier d'Arolla, Switzerland. The plan view shows the main inputs and transport processes occurring on the glacier surface. Cross-sections 1-3 summarise medial moraine formation. Cross-section 4 shows 9 the relationship between glacier structures and debris deposition near the glacier snout. [Reproduced with permission from: Goodsell et al. (2005) 9

Journal of Glaciology, 51, figure 9, p. 144]

Figure 7.12 Schematic summary of contemporary debris transport processes at Haut Glacier d'Arolla, Switzerland. The plan view shows the main inputs and transport processes occurring on the glacier surface. Cross-sections 1-3 summarise medial moraine formation. Cross-section 4 shows 9 the relationship between glacier structures and debris deposition near the glacier snout. [Reproduced with permission from: Goodsell et al. (2005) 9

Journal of Glaciology, 51, figure 9, p. 144]

Figure 7.13 Thrusts exposed in a lateral cliff at the glacier Midre Lovenbreen, Svalbard. Ice flow is right to left. Numerous low-angle thrusts can be seen rising from the bed towards the ice surface.

[Photograph: N.F. Glasser]

Figure 7.13 Thrusts exposed in a lateral cliff at the glacier Midre Lovenbreen, Svalbard. Ice flow is right to left. Numerous low-angle thrusts can be seen rising from the bed towards the ice surface.

[Photograph: N.F. Glasser]

ice to move up and over the colder ice in front (Figure 7.13). Thrusts may penetrate throughout the thickness of a glacier or terminate englacially as blind thrusts. Thrust planes often exploit structural weaknesses within the ice such as the traces of former crevasses. They may occur singularly and extend considerable distances across an ice margin (Figure 7.13), or in anastomosing networks in which each thrust has a short arcuate outcrop on the surface of the glacier. The angle between the glacier bed and the thrust plane is generally low (30-40°), although high-angle thrusts (40-80°) have been documented in surge-type glaciers. Thrusting is a particularly important mechanism within surging glaciers.

Debris entrainment into thrusts occurs in a variety of different ways, varying from incorporation of thin layers of debris-rich ice to large rafts of frozen sediment that are frozen to the glacier bed and elevated along thrust planes (Figure 7.14). Debris-rich thrusts may penetrate the full depth of a glacier so that the debris crops out on the glacier surface, or they may terminate englacially (Figure 7.14). Meltwater can also rise along thrusts, elevating substantial quantities of glaciofluvial sediment to the ice surface (Figure 7.15).

The transfer of basal debris into an englacial or supraglacial position is particularly important in determining the depositional processes that operate within a glacier (see Section 8.1). The presence or absence of thrusts, for example, is an important control on its debris structure. Thick englacial and supraglacial debris concentrations can be created by thrusting and these can have important implications for the release of debris from the glacier. Thrusts are comparatively rare in

Figure 7.14 The terminal zone of a Svalbard valley glacier, showing debris arranged on the ice surface in mounds and pinnacles as a result of ice-structural control. The debris has been elevated along thrusts within the ice. [Photograph: N.F. Glasser]

Figure 7.15 A pinnacle of debris on the surface of Marthabreen, Svalbard. This photograph illustrates neatly how different populations of sediment can occur in close proximity, with well-rounded cobbles and boulders melting out of a thrust and mixing with the supraglacial material on the ice surface. [Photograph: N.F. Glasser]

warm-based glaciers and thick basal debris layers are unusual because basal melting prevents their development. As a result, debris transport in temperate glaciers is dominated by high-level (passive) transport. In contrast, polythermal glaciers often have well-developed thrust planes. The result is that these glaciers have thicker englacial debris concentrations and the supraglacial debris at the glacier snout contains a mixture of debris transported at both high and low levels. The subglacial debris layer may also be thicker due to regelation in zones of thermal transition between warm- and cold-based ice. Glaciers that are completely cold-based throughout are dominated by high-level debris transport, although subglacial debris layers can develop.

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